THE
National Ignition Facility—the largest laser in the world—is a project
of extremes. "Very" tends to be attached to descriptions about it:
the facility is very large (the size of a sports stadium), the laser's
target is very small (the size of a BB-gun pellet), the laser system
is very powerful (equivalent to 1,000 times the electric generating
power of the U.S.), and each laser pulse is very short (a few billionths
of a second). All these extremes converge in the final action that
occurs in a 10-meter-diameter aluminum sphere, which is the target
chamber. "The entire system is basically
a target shooter, with the target chamber being the business end
of the system," says NIF project manager Ed Moses. Once the pulses
from the laser's 192 beams have been amplified, shaped, and smoothed,
they must pass through the final optics assemblies (FOAs) mounted
on the outer surface of the chamber. The FOAs—which contain frequency
conversion crystals, vacuum windows, focus lenses, diffractive optics,
and debris shields—convert the pulses from infrared to ultraviolet
light and focus the light on the target. (See the box below.) All
192 pulses then focus their total energy of 1.8 megajoules on the
target—a gold cylinder holding a 2-millimeter capsule containing
deuterium and tritium, two isotopes of hydrogen. Fusion—creating
on a minute scale the extreme temperatures and pressures found inside
stars and detonated nuclear weapons—is the goal. Experiments performed on
NIF will be essential to the Department of Energy's Stockpile Stewardship
Program, which has the task of ensuring the safety and reliability
of the nation's nuclear stockpile. NIF will also have basic science
applications in such areas as astrophysics, hydrodynamics, and material
properties and will forward the scientific pursuit of fusion energy.

Pulsing
the System

Generating
enough laser energy to cause fusion, thereby simulating
the goings-on in the Sun and stars, is
an exacting process. From start to
finish, each pulse of laser light must travel 450 meters
before it reaches the target. That pulse begins humbly
in the master oscillator system. A small fiber-ring
oscillator generates a weak, single-frequency laser
pulse
on the order of a nanojoule. That pulse is launched
into an optical fiber system that amplifies and splits
it until there are 192 10-joule pulses.The pulses enter
the main laser system, where each
light pulse makes four passes in a beampath of mirrors,
lenses, amplifiers, switches, and spatial filters. This
multipass concept was one of the design breakthroughs
of NIF. Without it, the facility would have had to be
over
a kilometer long for the pulses to gain the required
energy. In its multipass journey, each laser light pulse
bounces
off the equivalent of 54 mirrors and goes through the

equivalent of 2 meters of glass. Each pulse is reflected
off a deformable mirror to correct for aberrations that
accumulate in the beam because of minute distortions
in the optics. The mirror uses an array of actuators
to create a surface that will compensate for the accumulated
wavefront errors. Once the beams
have completed their passes through the main laser system,
they proceed to two switchyards on either side of the
target chamber. The switchyards take the 192 beams—which
up to now have been traveling in bundles of 8 beams,
4 high and 2 across—and split them into quads of 2-by-2
arrays of beams. The quads are "switched" into a radial,
three-dimensional configuration around the sphere. Just
before entering the target chamber, each quad of pulses
passes through a final optics assembly, where the pulses
are converted from infrared to ultraviolet light and
focused onto the target. The entire journey takes 1.5
microseconds.

What
Is Required of the ChamberThe
target chamber, the largest single piece of equipment for NIF, is
a 118,000-kilogram sphere made of aluminum alloy 5083—the same alloy
used to build ship superstructures. It has a diameter of 10 meters
and a wall thickness of 11 centimeters. The chamber must provide
a vacuum environment down to 10–6 torr and shield personnel and
surrounding areas from neutrons and gamma rays. And when it's ready
for experimental use, it must have 48 FOAs and nearly 100 diagnostic
instruments mounted on its surface. When the chamber was designed
in 1993, the design engineering team—led by Livermore's Vic Karpenko
and Sandia National Laboratories' Dick Wavrik—consulted with laser
scientists, optical experts, target physicists, laser physicists,
and facility designers at Lawrence Livermore and Los Alamos national
laboratories, the University of Rochester, and the Defense Threat
Reduction Agency to come up with design requirements. The list of
requirements included laser beams synchronized to arrive at the
target simultaneously, fixed focal plane distances from the final
optics to the targets, stringent vibration stability, and easy ingress
and egress for systems that transport, hold, and freeze the tiny
targets. The target chamber designers also had to consider space
and cost constraints. Says Rick Sawicki, the laser area integration
manager, "The world has made lots of spheres in the past, but all
the requirements added together made the NIF target chamber a very
challenging project, from an engineering perspective."Requirements
for the entire experimental system also affected the chamber and
its subsystems. For example, the lasers must point at the target
with extreme precision—on the equivalent of touching a single human
hair from 90 meters away with the point of a needle. "Overall,"
says Sawicki, "we must deliver 1.8 megajoules of energy to the target
with a 50-micrometer pointing stability on the target. That means
we must accurately and stably point all laser beams and hold the
target stable. Fifty micrometers is about the thickness of a sheet
of paper, so that's how little wiggle room we have for any vibration
in the system. Achieving that alignment on a table-top laser is
one thing. Achieving it on a system the size of NIF . . . that's
a huge challenge!"The NIF teams analyzed all
NIF structures to determine whether they could collectively meet
the requirements. That analysis pointed to the target chamber as
an important contributor to vibration. As a result, the target is
not supported by the chamber but by a target positioner attached
directly to the floor of the facility. Design features were implemented
to permit the positioner to pass through the wall of the target
chamber without coupling to the chamber's vibration, yet still maintain
vacuum continuity. Throughout the facility, other steps were taken
to dampen vibration and add stability. Concrete floors—nearly 2
meters thick in the Target Area Building and 1 meter thick in the
Laser Building—help deaden stray vibrations. The target chamber
is supported on a thick concrete pedestal and connected to the building
floors at its waist to minimize vibration-induced motion. The Laser
and Target Area Buildings will be temperature-controlled to 0.3 degrees Celsius
to maintain laser positioning. Sophisticated, low-vibration air-handling
systems have been installed and are being activated.

At
the National Ignition Facility, the Laser Building holds the
two laser bays, which house most of the components of the main
laser system; and the Target Area Building is divided into the
switchyards, the target diagnostic areas, and the target area.
The target area, a circular space, contains the target chamber
and its attendant equipment.

Moving
Right Along Work on the target chamber
has continued apace since the chamber was lowered into the Target
Area Building nearly two years ago (see S&TR, September
1999, Target
Chamber's Dedication Marks a Giant Milestone). Once the chamber
was settled onto its massive concrete pedestal, workers used hydraulic
jacks, roller assemblies, shims, and anchor bolts to align the chamber
and establish its proper elevation and tilt. Then the chamber was
leak-tested with helium gas. This testing had to be accurate because
all the weld joints are covered by shielding material, which prohibits
leak repairs. Next, the chamber was prepared for its shielding,
a 40-centimeter-thick skin of gunite—a mixture of cement, sand,
and water similar to that used to line swimming pools. The gunite
was combined with 0.1 percent boron, a neutron-absorbing, activation-limiting
material. Some 200 tons of the mixture was sprayed onto the chamber
surface, which was then sealed with epoxy paint. NIF workers then
opened the more than 70 ports for the FOAs and conducted a precision
survey to pinpoint where all the laser beams would intersect. "With all that concrete,
we expected the chamber to sag somewhat," says Sawicki. That sagging
would throw off the beam angles. Sagging also might be compounded
by mounting the FOAs, which will add another 200 tons to the structure.
Precision surveys have been performed to determine this impact as
well. "Once everything is in place," says Sawicki, "we will make
our final adjustments to the angle of the FOAs with simple spacers
that can be accurately machined." In the meantime, conventional
construction throughout the facility has proceeded to 96 percent
completion as of February 2001. Since the first of the year, both
laser bays have been certified for clean room protocols; and vessel
setting, steel framework fabrication, and installation of beampath
infrastructure have begun. All in all, more than 11,500 metric tons
of steel has been erected and more than 56,000 cubic meters of concrete
has been poured.

At
the target chamber exterior, the surface of the vessel is prepared
for an application of gunite. The shielding material is specially
formulated to absorb neutrons and minimize radioactive induction
in the aluminum chamber.

What's
Next?In February
2001, leak-testing was completed, and the target chamber was officially
"in acceptance," that is, ready to accept the final optics assemblies,
utilities, and diagnostics. "The chamber was designed for the lasers,
the diagnostics, and the Target Area Building," notes Moses. "Completing
it and putting it in place was an important stepping stone in building
the project." In both the Laser and Target Area Buildings, the next
major task is to install the beampath enclosures that connect the
target chamber to all of the other vessels in the facility and to
connect these enclosures to the utility systems (such as vacuum,
helium, argon, compressed air, and water). All this will be accomplished
while maintaining Level 100 cleanliness conditions inside the enclosures.
Elsewhere in the facility,
80 percent of the large components of the beampath infrastructure
(such as vacuum vessels, support structures, beam tubes, and beam
enclosures) have been procured and are either on the way or on site
being installed. Over the next couple of years, the project will
be making nearly $1 billion in procurements of special equipment
and putting it all together inside the space of the beampath enclosures.
"The design of the facility is essentially done," Moses says. "Now,
we need to turn from being an organization primarily focused on
design and engineering to an organization focused on procurement,
installation, and commissioning of the facility. That'll be our
next big challenge."—Ann
Parker